docs: def to_string(self, value):
          """
          In python3, json.dumps() cannot sort keys of different types,
          so preconvert None to 'null'.
          """
          self.enforce_type(value)
          if isinstance(value, dict) and None in value:
              value = value.copy()
              value['null'] = value[None]
              del value[None]
          return super(Dict, self).to_string(value)